The regulations are something of a grey area here. As I read them, the tread of the tyre must not protrude from the widest part of the arch which on The Zed is at the top, so it's O.K to have the tyre wall sticking out.
I'll take a photo of The Roadster which recently passed its MOT which illustrates how far you can go.
These are obviously not standard Rays, but I've calculated the amount of protrusion is equivalent to having 27/28mm spacers on Rays. Oh yeah, standard suspension on this Zed.
Here's what 25mms look like on the front once again not standard rays but will give you some idea of how much they fill the arches, bad news is that they rub on occasions.
Pete